Such force transmitting devices are known to someone skilled in the art of valve controllers with hydraulic valve lash compensation and are embodied according to the architecture of the internal combustion engine. Thus, the so-called “overhead camshaft,” also known as “OHC,” construction for a valve drive with a camshaft in the cylinder head for the most part uses a slaved cup tappet, valve lifter or rocker arm, as well as a stationary pivot bearing for finger levers, each with hydraulic valve lash compensation.
In addition, such force transmitting devices also find multiple uses in the so-called “overhead valve” valve drive arrangement known in short as “OHV” for predominantly large-volume internal combustion engines embodied as V engines. In the OHV arrangement, the valve drive is characterized by a camshaft, which is mounted in the engine block of the internal combustion engine in the vicinity of the crankshaft and whose cam lobes are picked up by longitudinally moving tappets equipped for the most part with hydraulic valve lash compensation as force transmitting devices and are converted into a lifting motion of the corresponding tappet connected to the cam. The lifting motion of the tappet is typically transmitted via a tappet rod, which actuates a rocker arm mounted in the cylinder head of the internal combustion engine, to one or more gas-exchange valves allocated to the tappet.
The known advantages of a hydraulic and thus automatic valve lash compensating device include, in particular, the elimination of the valve lash setting during the initial installation and maintenance of the internal combustion engine, its quiet running, and favorable exhaust gas emissions. However, these advantages can be realized completely only under the prerequisite that the hydraulic valve lash compensating device is operational or ready to operate in all operating states, including when the internal combustion engine is stopped and when started. The essential basis here is obviously an adequate supply of hydraulic medium to the valve lash compensating device. For this purpose, the hydraulic medium is fed during the operation of the internal combustion engine from a hydraulic medium pump via supply lines to a compensating piston of the valve lash compensating device, with the compensating piston defining a hydraulic cushion of a working space used for transmitting movements and forces. The working space is variable in volume, because the compensating piston is always aiming to adjust the height of the hydraulic cushion enclosed by the working space, so that mechanical play in the valve drive is eliminated during the lift-free base circle phase of the cam. The compensating piston typically has a hollow cylindrical shape and encloses a hydraulic medium reservoir, which supplies the working chamber with hydraulic medium via a non-return valve during valve lash compensation movements, i.e., when the working chamber is expanding. Here, it has proven to be useful if the volume of the hydraulic medium reservoir equals a multiple of the volume of the working chamber, in order to reliably rule out undesired suctioning of air or gas bubbles into the working chamber under all operating conditions of the internal combustion engine.
In connection with this, a starting process of a cold internal combustion engine, which typically would have been turned off with one or more open gas-exchange valves, represents an especially critical operating state, so that the compensating piston of the corresponding valve lash compensating devices are lowered partially or completely due to hydraulic medium being largely forced from the working chamber under the effect of the gas-exchange valve spring and according to the duration of the intermediate standstill phase of the internal combustion engine. In addition, the hydraulic medium pump delivers no or only an inadequate hydraulic medium volume flow to the compensating piston during the starting process. In this respect, essentially the sole task of the hydraulic medium reservoir is to completely satisfy the considerable hydraulic medium requirements of the working chamber during its expansion from the lowered position of the compensating piston into its working position.
An inadequately large or an inadequately filled hydraulic medium reservoir would necessarily lead to suctioning of air or gas bubbles into the working chamber. The consequences of a working chamber containing air or gas bubbles for the valve drive function during starting and operation of the internal combustion engine are known to someone skilled in the art and are perceived audibly as disruptive as so-called valve tapping primarily due to high contact velocities of the gas-exchange valve during their closing process.
The requirement for a sufficiently large hydraulic medium reservoir also stands increasingly in conflict with the goal of further reducing the installation space and/or the mass of the force transmitting device or for expanding the functionality of the force transmitting device while not changing the installation space. The latter case includes, in particular, variable force transmitting devices, which are embodied as reversible force transmitting devices and which each transmit strokes from different cams selectively to the gas-exchange valve according to the switching state of their coupling means and/or can completely mask the stroke of one cam. Thus, for example, for switchable push rod valve trains in an OHV arrangement, it is typical to interleave cam follower parts, which can be displaced longitudinally and which can be coupled with each other, one in the other, so that the outer and connection geometry of the cam follower can remain essentially unchanged. However, this normally requires a reduction in installation space of the hydraulic valve lash compensating device and consequently a reduction in volume of the hydraulic medium reservoir enclosed by the compensating piston with the previously explained risks and the consequences of an insufficient hydraulic medium supply to the working chamber.
In the state of the art, there have already been approaches for solving the problems named above. Thus, for example, in U.S. Pat. No. 4,462,364, which is considered a class-defining invention, as well as in DE 197 54 016 A1, retaining means have been proposed, which should prevent the draining of the hydraulic medium reservoir. In the non-pressurized state of the hydraulic medium supply, these retaining means completely enclose the hydraulic medium reservoir found in the compensating piston, whereby a partial or complete loss of hydraulic medium from the compensating piston can be prevented, especially for an installation position of the force transmitting device suitable for the force of gravity. However, simultaneously, the hydraulic medium volume made available to the working chamber is restricted by the size of this internal hydraulic medium reservoir. In this respect, these retaining means might not be suitable especially for switchable cam followers with installation space-reduced compensating pistons, in order to guarantee a complete refilling of the working chamber primarily during the starting phase of the engine.